Related Party Transactions and Investments in Non-Consolidated Entities - Management fees and interest income from loans (Details) - USD ($)	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Related Party Transactions and Investments in Non-Consolidated Entities
Asset management fees, low end of range (as a percent)	1.00%
Asset management fees, high end of range (as a percent)	5.00%
Notice period for cancellation of applicable contracts	30 days
Revenue	$ 45,000	$ 121,000
Revenue, Product and Service [Extensible List]	us-gaap:AssetManagement1Member	us-gaap:AssetManagement1Member

High end of the range of the asset management fee as a percentage of collected rents.

No definition available.

Name:	fsp_AssetManagementFeesPercentageHighEndOfRange
Namespace Prefix:	fsp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Low end of the range of the asset management fee as a percentage of collected rents.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the notice period for cancellation of applicable contracts.

+ References

No definition available.

+ Details

Name:	fsp_NoticePeriodForCancellationOfAssetManagementFeeContract
Namespace Prefix:	fsp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_RelatedPartyTransactionsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Indicates type of revenue from product and service. Includes, but is not limited to, revenue from contract with customer and other sources.

+ References

No definition available.

+ Details

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Data Type:	srt-types:extensibleListItemType
Balance Type:	na
Period Type:	duration